In recent years, bioethanol obtained by fermenting biomass to produce an aqueous ethanol solution and dehydrating and purify the ethanol solution has been attracting attention for use as an energy source. However, because ethanol and water form an azeotropic mixture, it is impossible to dehydrate and purify an aqueous ethanol solution to a purity of 96% by weight or more by ordinary distillation operations. Then, in order to obtain high purity ethanol with a minimum ethanol content of 99% by weight, azeotropic distillation using an entrainer, e.g., cyclohexane, has been practiced. On the other hand, a separation membrane is capable of easily separating an organic vapor mixture of water and ethanol that forms an azeotropic mixture by taking advantage of the difference in permeability between the components. Separating an aqueous ethanol solution into ethanol vapor and water vapor by the use of a separation membrane is a promising dehydration method for obtaining high purity ethanol as an approach to the establishment of an energy saving system replacing the azeotropic distillation method.
In general, organic vapor separation using a gas separation membrane module is performed as follows. A liquid mixture containing an organic compound is heated to form an organic vapor mixture, which is fed into a gas separation module from a mixed gas inlet. While flowing in the module in contact with a separation membrane, the organic vapor mixture is separated into permeate vapor (vapor having passed through the membrane) and retentate or non-permeate vapor (vapor having been rejected by the membrane). The permeate vapor is recovered from a permeate outlet, and the retentate vapor is recovered from a retentate outlet. The permeate vapor is rich in a component having a high permeation rate through the membrane (hereinafter also referred to a readily permeable component), while the retentate vapor has a diminished content of the readily permeable component. As a result, the organic vapor mixture is separated into permeate vapor rich in the readily permeable component and retentate vapor with a scarce readily permeable component.
Patent document 1 (see below) proposes using a gas separation membrane made of polyimide in a organic vapor dehydration process in which an aqueous solution containing an organic substance is vaporized into a gas mixture containing organic vapor and water vapor, and the water vapor is selectively excluded to give a high concentration organic solvent. The polyimide used to make the gas separation membrane is composed of an aromatic tetracarboxylic acid skeleton as a tetracarboxylic acid component derived from 3,3′,4,4′-biphenyltetracarboxylic acid and/or 2,3,3′,4′-biphenyltetracarboxylic acid and an aromatic diamine backbone as a diamine component derived from at least one diamine selected from the group consisting of 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ether, and diaminodiphenylmethane.
However, the polyimide hollow fiber membrane made of the aromatic polyimide disclosed in Patent document 1 has a water vapor permeance (P′H2O) of 1.47×10−3 cm3(STP)/cm2·sec·cmHg at the highest and a water vapor to ethanol vapor permselectivity (i.e., permeance ratio: P′H2O/P′EtOH) of no more than 22. In particular, the polyimide hollow fiber membrane made of an aromatic polyimide composed of a tetracarboxylic acid skeleton derived from 3,3′,4,4′-biphenyltetracarboxylic acid and a diamine backbone derived from 60 mol % of 3,4′-diaminodiphenyl ether and 40 mol % of 4,4′-diaminodiphenyl ether has a water vapor permeance (P′H2O) of 1.24×10−3 cm3(STP)/cm2·sec·cmHg.
Patent document 2 (see below) discloses a soluble aromatic polyimide prepared by polymerizing and imidizing a tetracarboxylic acid component mainly comprising a biphenyltetracarboxylic acid and an aromatic diamine component comprising 25 to 100 mol % of a 2,2-bis[(aminophenoxy)phenyl]propane (BAPP) in an organic solvent, such as a phenol compound.
Patent document 3 (see below) discloses a gas separation membrane mainly comprising an aromatic polyimide obtained by copolymerizing a tetracarboxylic acid component mainly comprising a biphenyltetracarboxylic acid and a diamine component mainly comprising a 1,4-bis(4-aminophenoxy)benzene (TPEQ) and a diaminodiphenyl ether (DADE) and an aromatic polyimide obtained by copolymerizing a tetracarboxylic acid component mainly comprising a biphenyltetracarboxylic acid and a diamine component mainly comprising a 1,4-bis(4-aminophenoxy)benzene (TPEQ) and a 1,3-bis(aminophenoxy)benzene (TPER). The gas separation membrane is described as exhibiting high levels of heat resistance, water resistance, gas separation performance (permeability to water vapor and water/organic substance permselectivity) and, in particular, resistance to a high-temperature liquid mixture of water and an organic substance.    Patent document 1: JP 63-267415A    Patent document 2: JP 02-222716A    Patent document 3: JP 02-222717A